1. Field of the Invention
The present invention relates to velocity models and three dimensional image construction and more particularly to the construction of three dimensional images using a grid of two dimensional depth sections.
2. Related Prior Art
The acquisition of a seismic line that is a true dip line is the exception rather than the rule. The result is that most reflectors originate out of the vertical reflection plane, and the problem of out of the plane events increases in severity with depth and structural complexity. A three dimensional prestack depth migration is needed to properly image all the reflectors. It is an expensive process to generate a three dimensional velocity model through repetitive three dimensional seismic migrations. Also, picking from stacked sections is extremely difficult in complex areas because the primary reflections are badly distorted. In contrast, two dimensional pre-stack depth migration produces a stronger image in these situations and is becoming cheaper to produce. Pre-stack two dimensional depth migration does the optimum job of migrating data that has dip along the line, but it does not image the cross-dipping events correctly.
There are many methods for producing three dimensional models from two dimensional data. Examples of methods which may be employed in working with three dimensional data are contained in U.S. Pat. Nos. 4,736,347 and 4,241,429.
U.S. Pat. No. 4,736,347 titled "Multiple Stacking and Spatial Mapping of Seismic Data" (Bernard Goldberg et al.) relates to a method for determining the dip of subsurface formations and the apparent acoustic velocity Seismic traces are stacked in a plurality of orthogonal measures to form multiple stacked traces at a positive offset. The stacking process determines the apparent velocities as functions of the travel time at the positive offset. The interval acoustic velocity of the first layer is then determined from knowledge of surface topography, source-receiver offset, two-way travel times and the first reflector apparent velocities. The first layer velocity information enables the incident and emergent angles of the raypaths at the surface to be calculated, as well as enabling the dip angles and spatial coordinates of the reflection points on the first reflecting boundary to be determined. Seismic data corresponding to the second reflecting boundary are then mapped spatially to the first reflecting boundary by ray tracing and by calculating the apparent velocities at the first boundary. The process is repeated for each succeedingly deeper boundary. The derived acoustic velocity model of the earth is displayed as a stacked seismic section in spatial coordinates This process may be applied to obtain earth models and seismic sections in both two and three dimensions.
U.S. Pat. No. 4,241,429 titled "Velocity Determination and Stacking Process from Seismic Exploration of Three Dimensional Reflection Geometry" (Marvin G. Bloomquist et al.) relates to a method for determining the dip and strike of subsurface interfaces and average propagation velocity of seismic waves. In seismic exploration, linear, multiple fold, common depth point sets of seismograms with three dimensional reflection geometry are used to determine the dip and strike of the subsurface reflecting interfaces and the average velocity of the path of the seismic energy to the reflecting interface. The reflections in each set appear with time differences on a hyperbola with trace spacings determined by the source receiver coordinate distance along the lines of exploration. The offset of the apex of this hyperbola is determined from a normal moveout velocity search of the type performed on two dimensional common depth point (CDP) sets. This search identifies the correct stacking velocity and hyperbola offset which are used to determine dip, strike and average velocity.